A Genetically-Engineered Thyroid Gland Built for Selective Triiodothyronine Secretion.

Thyroid hormones (thyroxine, T4, and triiodothyronine, T3) are indispensable for sustaining vertebrate life, and their deficiency gives rise to a wide range of symptoms characteristic of hypothyroidism, affecting 5-10% of the world's population. The precursor for thyroid hormone synthesis is thyroglobulin (Tg), a large iodoglycoprotein consisting of upstream regions I-II-III (responsible for synthesis of most T4) and the C-terminal CholinEsterase-Like (ChEL) domain (responsible for synthesis of most T3, which can also be generated extrathyroidally by T4 deiodination). Using CRISPR/Cas9-mediated mutagenesis, we engineered a knock-in of secretory ChEL into the endogenous TG locus. Secretory ChEL acquires Golgi-type glycans and is properly delivered to the thyroid follicle lumen, where T3 is first formed. Homozygous knock-in mice are capable of thyroidal T3 synthesis but largely incompetent for T4 synthesis such that T4-to-T3 conversion contributes little. Instead, T3 production is regulated thyroidally by thyrotropin (TSH). Compared to cog/cog mice with conventional hypothyroidism (low serum T4 and T3), the body size of ChEL-knock-in mice is larger; although, these animals with profound T4 deficiency did exhibit a marked elevation of serum TSH and a large goiter, despite normal circulating T3 levels. ChEL knock-in mice exhibited a normal expression of hepatic markers of thyroid hormone action but impaired locomotor activities and increased anxiety-like behavior, highlighting tissue-specific differences in T3 versus T4 action, reflecting key considerations in patients receiving thyroid hormone replacement therapy.